Variable displacement pumps, such as those employed in hydrostatic transmissions, are adapted to have the displacements thereof varied between minimum and maximum levels upon adjustment of servo-systems connected to the pumps which are adapted to actuate swash plates of the pumps in a conventional manner. U.S. Pat. No. 3,996,743, assigned to the assignee of this application and issued to Cyril W. Habiger on Dec. 14, 1976, discloses a conventional control circuit for controlling the displacement of pumps of this type.
The control circuit comprises an underspeed actuator which is connected to the servo-systems and further connected to a venturi to receive a differential fluid pressure signal therefrom which is proportional to the speed of a vehicle's engine to automatically control the underspeed actuator for automatically varying the displacement of the pumps under certain conditions of engine operation. The control circuit further comprises a directional control valve movable to a vent and override position to equalize the differential fluid pressure signal whereby the automatic operation of the underspeed actuator may be overriden.
When the control valve is in this position, pressurized fluid is blocked from communicating with replenishing and relief valves of the pump system and also to normally "on" brakes of the vehicle which must be released by fluid pressure. When the control valve is moved from its vent towards its maximum speed position, pressurized fluid is communicated to the brakes to release them and also to the replenishing and relief valves of the pump system to condition the pumps for operation. Simultaneously therewith, the above-mentioned differential fluid pressure signal is re-established to permit the underspeed actuator to control displacement of the pumps automatically, should the need arise.
It has proven desirable, particularly in the application of the control circuit of the present invention to hydrostatic transmissions, to construct and arrange an override control valve as a separate unit from a main control valve which controls the venting and primary control functions of the circuit. In addition to rendering the circuit efficient for expeditious and close control by the operator, the respective control valves and associated components of the control circuit may be arranged for efficient servicing. Also, the main control valve can be maintained operational in its "run" position upon movement of the override control valve from its closed position to its open, override position and back to its closed position.